U.S. Pat. No. 4,476,459 discloses a system comprising a transmitter antenna and a receiver antenna mounted within an interrogation zone, formed by the space around the said transmitter and receiver antennas. This transmitter, receiver system is used for detecting the passage of a resonant circuit, through the interrogation zone, by using the following function sequence:                1. The transmitter delivers a short electromagnetic pulse to the interrogation zone.        2. If a resonant circuit is present in the interrogation zone, it receives an electromagnetic energy pulse from the transmitter.        3. Having received the said electromagnetic energy pulse, the resonant circuit resonate at its resonance frequency and thereby transmits energy to the interrogation zone receiver antenna. The signal transmitted from the resonant circuit is decaying in time, and this time decay is used in the receiver for detecting the presence of a resonant circuit, and reduce the number of erroneous detections, which might originate in the electromagnetic noise from the environment.        
EP 0 743 540 A2 discloses a system being adapted to detect the presence of foreign objects inside a robbery-prevention security antechamber, e.g. commonly used in banks. The antechamber consists of a plurality of radiofrequency emitting elements, associated with a plurality of receiving units. The six walls of the antechamber are preferably made from electrically conducting materials, which can reflect microwaves in the range of 10 GHz. The antechamber is considered as a resonant cavity, where the distribution of the electromagnetic field depends on the geometric dimensions of the cavity (the antechamber) and the materials in the cavity. A modification of the situation in the antechamber, e.g. by the passage of a person, will change the distribution of the standing wave electromagnetic field. The modification can be detected by analyzing the standing wave electromagnetic field before passage of a person and compare it to the field after passage of a person. The transmitters and receivers are positioned in a two dimensional grid in alternating grid points in the ceiling. An important feature of the transmitters is that they have a sufficient spectral purity, to obtain stable standing wave analysis. The measurement of the standing wave field in the antechamber is carried out by enabling the transmitters one-by-one, and carry out a simultaneous recording of the field using all receivers. These measurements are then compared to what is obtained when a person passes the chamber using the same said measuring procedure, and thereby a detection of a foreign object in the antechamber is obtained.
U.S. Pat. No. 6,288,640 B1 discloses a system comprising a plurality of sensors and a corresponding plurality of receivers. The sensors receive an FM signal broadcast by a transmitter, preferably a commercial FM radio station. Each receiver receives, via the associated sensor, radio frequency signals comprising a multiplicity of transmissions at different frequencies, within a predefined frequency spectrum. The receiver detects the radio frequency signals and computes, for each of a plurality of successive time intervals and for each of the transmission frequencies, a measurement of the signal amplitude over the time interval, compares the signal amplitude measurement with at least one threshold and, if the threshold is exceeded, generates a potential alarm signal. An alarm signal might be generated if a preset number of a multiplicity of the transmission frequencies have detected an alarm. No comparison between two independently transmitted and received signals is performed.